Muscle contraction comes in three basic flavors: concentric, eccentric and isometric, with each type asking for a different demand on your muscle fibers. Pilates, a form of biomechanically-correct calisthenic exercise devised in the 1920s and still in use today, uses all three types of muscle activation for different purposes – shortening, lengthening or stabilizing a muscle or limb. Although isometric contraction can play a part in Pilates exercises, it usually comes into play in the final set of a series of movements to bring your muscles to fatigue. You can experience all three types of contraction while performing the Pilates frog exercise, where the contraction types flow from one to the next.

Pilates Frog Exercise

Set up for the Pilates frog exercise by lying on your back on a mat and placing a 9-inch playground ball under your hips to create a supported bridge position. Bend your knees toward your chest and make a “V” shape with your feet, allowing your knees to splay out to look like a frog on its back. The first muscle action involves extending both legs forward at a 45-degree angle from your mat. You'll look like a human teeter-totter. The second muscle action consists of tucking the knees toward the chest to return to your frog position. Perform 10 to 20 leg-extend and bend repetitions and then finish with a 10-second hold of the teeter-totter position.

Eccentric

Eccentric contraction requires a muscle to lengthen as it manages a load. This happens in the first movement of the frog exercise, when your legs extend forward to their full length creating a 45-degree angle with the floor. Imagine your body as a human teeter-totter – if one end becomes longer, it requires more work to stabilize that longer end. In this movement, your quadriceps muscles use eccentric contraction to extend like a rubber band while managing the weight of your leg.

Concentric

Concentric contraction involves shortening a muscle as it performs work. Think of a biceps curl. When you bend your elbow to lift a weight, your biceps muscle shortens as it works to lift the weight. In the frog exercise, concentric contraction occurs in the second movement, when you bend your knees toward your chest. Since your legs began this movement at a 45-degree angle to the floor, they weigh quite a bit. Your hamstring muscles shorten as they perform the work of bending your leg and moving this heavy weight.

Isometric

Isometric contraction entails the static hold of a workload, instead of a movement. This happens in the last portion of the frog exercise, when you extend your legs at a 45-degree angle from the floor for a 10-second hold. Your abdominal muscles work in isometric contraction here to manage the weight of your legs. Pilates teachers often add this hold to work your muscles to full fatigue at the end of an exercise series. They rarely begin with an isometric hold because it is likely to fatigue your muscles and deteriorate the quality of movement during the eccentric and concentric phases of your exercises.

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About the Author

Based in Portland, Ore., Christine Binnendyk has written about health topics since 2001. She is the author of the book "Ageless Pilates" and her work has appeared in "SELF" magazine and "Pilates Pro." Binnendyk holds a Bachelor of Arts in journalism from the University of Connecticut and certifications from YogaFit, ACE, IDEA, Oregon School of Massage and the Pilates Studio of New York.